THE PROPERTIES OF DRILLING MUDS AT HIGH-PRESSURES AND HIGH-TEMPERATURES

This paper describes an experimental study of the rheological properties of various aqueous bentonite suspensions which resemble those oil-well drilling fluids, or muds, which are encountered in practice. Data are presented for systems which are termed 'concentrated' muds. 'thinned' muds and 'barite-loaded' muds, under the conditions which commonly occur during oil-well drilling operations, namely, at high pressures and high temperatures. Concentrated muds comprise of suspensions containing 7-10% (by mass) clay particles. The thinned variety contains similar quantities of clay particles but relatively large amounts of electrolytes. The inclusion of barite particles in these suspensions is a routine commercial means of increasing the density of the media. The data have been obtained by the use of a rolling-ball type rheometer, for performing rheological measurements at pressures up to 1400 bar (1 bar almost-equal-to 10(5) Pa) and at temperatures up to 140-degrees-C. The rheological response of these model drilling fluids is shown to approximate to those of Bingham fluids. The experimental data, which are reported in terms of two Bingham parameters (a yield stress and a plastic viscosity), show that the application of high pressure modifies the Bingham characteristic parameters in a way which is both temperature and mud composition dependent. The rheology of these muds is found to be shear-history dependent and the extent of this effect is described in terms of a 'ten minute gel strength'. This parameter provides a means of quantifying the thixotropic properties, or gel restructuring rate, of the muds. An induced volume change model, an application of the law of Corresponding States, has been introduced to describe the variations of the Bingham parameters (the yield stress and the plastic viscosity) of the concentrated muds as a function of pressure, temperature and clay content. The model is generally effective for the rationalization of the variation of the plastic viscosity. The model, however, presumes an equilibrium state for the system, and the rationalization of the yield stress data on this basis, particularly at low temperatures, is much less satisfactory. The data suggest that the characteristic gel restructuring times are long compared with the timescale of experiments and hence, complete gel formation may not have occurred during the timescale of our experiments, particularly at lower temperatures. Hence the full plastic yield strength potential was not achieved. At higher temperatures, it appears that the gel structure is more fully recovered within the experimental timescale and the inference is therefore that the gelation process is thermally activated. The data also suggested that thermally induced electrolyte dissolution may be responsible for certain features noted in the temperature dependence of the rheology. In addition, the densities and the sonic velocities of these muds at high pressures and high temperatures are described and discussed.